This invention relates to a resin composition, a porous film which is produced from the composition and is readily degraded by microorganisms under natural conditions, and a process for producing the film.
Porous films produced by compounding a polyolefin resin such as polyethylene or polypropylene resin with an inorganic filler or fillers such as calcium carbonate or barium sulfate, melting and filming the mixture, then stretching the resulting film have been used in many fields as various wrapping materials, medical supplies, clothing materials, etc. However, plastics represented by polyolefin resins are chemically so stable that used plastic products will keep their shape over a long period of time when left under natural conditions. In addition, they will become voluminous waste for their weight due to the low density of the plastics. Thus, disposal of used plastic products have become a serious object of public concern.
On the other hand, it has been conducted to mix an organic substance such as starch in polymers for the purpose of solving the problem of disposal of used plastic products. However, the resins themselves are not degraded, though the organic substance such as starch is degraded and the plastic products lose their shape. Hence, this technique of mixing the organic substance fails to completely solve the problem of disposal of used plastic products.
As plastics which are biodegradable by microorganisms such as bacteria and molds, there are illustrated chemically synthesized aliphatic polyester resins represented by poly-.epsilon.-caprolactone and aliphatic polyesters yielded by microorganisms such as a polyhydroxybutyrate polymer and a copolymer between hydroxybutyrate and hydroxyvalerate.
However, these biodegradable resins have the defects of low melting points, poor workability, expensiveness, etc. In order to improve their workability, it has been proposed to blend polyhydroxybutyrate with poly-.epsilon.-caprolactone (Japanese Unexamined Patent Publication No. H3-157450) or to add a filler such as calcium carbonate or starch to the aliphatic polyester to thereby control degradation rate or decrease the price (Japanese Unexamined Patent Publication No. H4-146952).
As to the former blending technique, however, only polyhydroxybutyrate is disclosed as a resin component to be mixed with poly-.epsilon.-caprolactone, as described above, and it is not disclosed to blend with a copolymer between hydroxybutyrate and hydroxyvalerate. As to the latter filler-adding technique, only polyhydroxybutyrate, poly-.epsilon.-caprolactone, etc. are disclosed as the aliphatic polyesters, and it is not disclosed, either, to use a copolymer between hydroxybutyrate and hydroxyvalerate.
In addition, the two techniques disclose using such resin compositions merely as materials for producing moldings of various shapes, and it is not disclosed to use the compositions as materials for producing porous films. In fact, the resin compositions are insufficient as materials for producing porous films due to their poor elongation upon filming and poor heat resistance.